1514.8.3 North America
The climate of North America (NA) is affected by the following phenomena: NAO (Section 14.5.1), ENSO (Section 14.4), PNA (Section 14.7.2), PDO (Section 14.7.3), NAMS (Section 14.2.3), TCs and ETCs (Section 14.6). The NAO affects temperature and precipitation over Eastern NA during winter (Hurrell et al., 2003). Positive PNA brings warmer temperatures to northern Western NA and Alaska in winter, cooler temperatures to the southern part of Eastern NA, and dry conditions to much of Eastern NA (Nigam, 2003). The PNA can also be excited by ENSO-related SST anomalies (Horel and Wallace, 1981; Nigam, 2003). The PDO is linked to decadal climate anomalies resembling those of the PNA. The NAMS brings excess summer rainfall to Central America and Mexico and the southern portion of Western NA (Gutzler, 2004). TCs also impact the Gulf Coast and Eastern NA (see Section 14.6.1). The AMM and AMO may affect their frequency and intensity (Landsea et al., 1999; Goldenberg et al., 2001; Cassou et al., 2007; Emanuel, 2007; Vimont and Kossin, 2007; Smirnov and Vimont, 2011). ETCs are also mainly responsible for winter precipitation, especially in the northern half of NA. See Table 14.3 for a summary of this information. A general surface warming over NA has been documented over the last century (see Section 2.4). It is particularly large over Alaska and northern Western NA during winter and spring and the northern part of Eastern NA during summer (Zhang et al., 2011b). There is also a cooling tendency over Central and Eastern NA (i.e., the ‘warming hole’ discussed in Section 2.4.1) during spring, though it is absent in lower tropospheric temperature (cf. Figure 2.25). The warming has coincided with a general decline in NA snow extent and depth (Brown and Mote, 2009; McCabe and Wolock, 2010; Kapnick and Hall, 2012). Consistent with surface temperature trends, temperature extremes also exhibit secular changes. Cold days and nights have decreased in the last half century, while warm days and nights have increased (see Chapter 2). These changes are especially apparent for nightly extremes (Vincent et al., 2007). It is unclear whether there have been mean precipitation trends over the last 50 years (Section 2.5.1; Zhang et al., 2011b). However, precipitation extremes increased, especially over Central and Eastern NA (see Section 2.6.2 and Seneviratne et al., 2012). Table 14.2 provides an assessment of GCM quality for simulations of temperature, precipitation, and main phenomena in NA’s regions. Regarding regional modelling experiments since AR4, biases have decreased somewhat as resolutions increase. The North American Regional Climate Change Assessment Program created a simulation suite for NA at 50-km resolution. When forced by reanalyses, this suite generally reproduces climate variability within observational error (Leung and Qian, 2009; Wang et al., 2009b; Gutowski, 2010; Mearns et al., 2012).Other regional modelling experiments covering parts or all of NA have shown improvements as resolution increases (Liang et al., 2008a; Lim et al., 2011; Yeung et al., 2011), including for extremes (Kawazoe and Gutowski, 2013). Bias reductions are large for snowpack in topographically complex Western NA, as revealed by 2- to 20-km resolution regional simulations (Qian et al., 2010b; Salathe Jr. et al., 2010; Pavelsky et al., 2011; Rasmussen et al., 2011). Thus there has been substantial progress since AR4 in understanding the value of regional modelling in simulating NA climate. The added value of using regional models to simulate climate change is discussed in Section 9.6.6. NA warming patterns in RCP4.5 CMIP5 projections are generally similar to those of CMIP3 (Figures AI.4 and AI.5, Table 14.1). In winter, warming is greatest in Alaska, Canada, and Greenland (Figures AI.12 and AI.16), while in summer, maximum warming shifts south, to Western, Central, and Eastern NA. Examining near-term (2016–2035) CMIP5 projections of the less sensitive models (25th percentile, i.e., upper left maps in Figures AI.12, AI.13, AI.16, AI.17, AI.20 AI.21, AI.24 and AI.25), the warming generally exceeds natural variability estimates. Exceptions are Alaska, parts of Western, Central, and Eastern NA, and Canada and Greenland during winter, when natural variability linked to wintertime storms is particularly large. By 2046–2065, warming in all regions exceeds the natural variability estimate for all models. Thus it is very likely the warming signal will be large compared to natural variability in all NA regions throughout the year by mid-century. This warming generally leads to a two- to four fold increase in simulated heat wave frequency over the 21st century (e.g., Lau and Nath, 2012). Anthropogenic climate change may also bring systematic cold-season precipitation changes. As with previous models, CMIP5 projections generally agree in projecting a winter precipitation increase over the northern half of NA (Figure 14.18 and AI.19). This is associated with increased atmospheric moisture, increased moisture convergence, and a poleward shift in ETC activity (Section 14.6.2 and Table 14.3). The change is consistent with CMIP3 model projections of positive NAO trends (Table 14.3; Hori et al., 2007; Karpechko, 2010; Zhu and Wang, 2010). Winter precipitation increases extend southward into the USA (northern portions of SREX regions 3 to 5; Neelin et al., 2013) but with decreasing strength relative to natural variability. This behaviour is qualitatively reproduced in higher resolution simulations (Figure 14.18). Warm-season precipitation also exhibits significant increases in Alaska, northern Canada, and Eastern NA by century’s end (Figures 14.18, AI.19, AI.22). However, CMIP5 models disagree on the sign of the precipitation change over the rest of NA (Figures AI.26 and AI.27), consistent with CMIP3 results (Figure 14.18; Neelin et al., 2006; Rauscher et al., 2008; Seth et al., 2010). One set of high resolution simulatons (Endo et al., 2012) shows a tendency towards more precipitation than either CMIP3 or CMIP5 models (Figure 14.18), suggesting the simulated warm-season precipitation change in the region may be resolution-dependent. Future precipitation changes associated with the NAMS are likewise uncertain, though there is medium confidence the phenomenon will move to later in the annual cycle (Section 14.2.3, Table 14.3). As there is medium confidence tropical cyclones will be associated with greater rainfall rates, the Gulf and East coasts of NA may be impacted by greater precipitation when tropical cyclones occur (Table 14.3). CMIP3 models showed a 21st century precipitation decrease across much of southwestern NA, accompanied by a robust evaporation increase characteristic of mid-latitude continental warming (Seager et al., 2007; Seager and Vecchi, 2010) and an increase in drought frequency (Sheffield and Wood, 2008; Gutzler and Robbins, 2011). When downscaled, CMIP3 models showed less drying in the region (Gao et al., 2012c) and an extreme precipitation increase, despite overall drying (Dominguez et al., 2012). CMIP5 models do not consistently show such a precipitation decrease in this region (Neelin et al., 2013). This is one of the few emerging differences between the two ensembles in climate projections over NA. However, the CMIP5 models still show a strong decrease in soil moisture here (Dai, 2013), due to increasing evaporation. In summary, it is very likely that by mid-century the anthropogenic warming signal will be large compared to natural variability such as that stemming from the NAO, ENSO, PNA, PDO, and the NAMS in all NA regions throughout the year. It is likely that the northern half of NA will experience an increase in precipitation over the 21st century, due in large part to a precipitation increase within ETCs.